Outer surface of a lift arm



FIG. 1 is an elevated rear perspective view of an outer surface of a lift arm showing a first embodiment of our new design, the broken line showing of the vehicle and tool is for illustrative purposes only, and forms no part of the claimed design;

FIG. 2 is an enlarged side view of the lift arm design of FIG. 1;

FIG. 3 is an elevated rear perspective view of an outer surface of a lift arm showing a second embodiment of our new design, the broken line showing of the vehicle and tool is for illustrative purposes only, and forms no part of the claimed design; and,

FIG. 4 is an enlarged side view of the lift arm design of FIG. 3. 

The ornamental design for an outer surface of a lift arm, as shown and described. 